Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material.
EP-A 0 901 040 discloses a substantially light-insensitive monosheet recording material comprising a support and a thermosensitive element containing a substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith and a binder, characterized in that said thermosensitive element further contains an unsaturated carbocyclic or heterocyclic stabilizer compound substituted with a —SA group where A is hydrogen, a counterion to compensate the negative charge of the thiolate group or a group forming a symmetrical or an asymmetrical disulfide and said recording material is capable of producing prints with a numerical gradation value defined as the quotient of the fraction (2.5−0.1)/(E2.5−E0.1) greater than 2.3, where E2.5 is the energy in Joule applied in a dot area of 87 μm×87 μm of the imaging layer that produces an optical density value of 2.5, and E0.1 is the energy in Joule applied in a dot area of the imaging layer material that produces an optical density value of 0.1.
EP-A 0 897 130 discloses a thermographic recording element having at least one image forming layer, said element comprising an organic silver salt, a reducing agent, and at least one of the substituted alkene derivatives of the general formulae (1) through (14):
wherein W is an electron attractive group, D is an electron donative group, H is hydrogen, the groups represented by W or D attached to the same carbon atom, taken together, may form a cyclic structure, the compound may assume either a trans or cis structure when both trans and cis structures are possible with respect to W or D, and two W groups in formula (14) form a cyclic structure.
U.S. Pat. No. 3,839,041 discloses a photothermographic element comprising a support having thereon photosensitive silver halide in association with an oxidation-reduction image-forming combination comprising (i) a heavy metal salt oxidizing agent with (ii) a reducing agent, a binder, and a stabilizer precursor which is a compound of the formula:
Wherein R3 is acetyl, propionyl, butyryl, pentanoyl, carboxy, cyano, aroyl containing up to 13 carbon atoms, or furyl; R5 is alkyl containing one to five carbon atoms, carboxy, aryl containing six to 12 carbon atoms, or hydrogen; R4 is alkyl containing one to five carbon atoms, carboxy, or hydrogen; n is 0 or 2.
Differences between substantially light-insensitive thermographic recording materials and photothermographic recording materials
The technology of substantially light-insensitive thermographic materials in which image formation is based on the reduction of organic silver salts is significantly different from that of photothermographic recording materials, despite the fact that in both cases the image results from the reduction of organic silver salts. However, this a superficial similarity masking the fact that the realization of the species which catalyze this reduction is completely different, being image-wise exposure of photosensitive silver halide-containing photo-addressable thermally developable elements in the case of photothermographic recording materials and image-wise heating of thermosensitive elements which do not contain photosensitive silver halide in the case of thermographic recording materials. This difference in technology is further underlined by the nature of the ingredients used in the two types of materials, the most significant difference being the absence of photosensitive silver halide and spectral sensitizing agents in substantially light-insensitive thermographic recording materials, but also reflected in the different reducing agents used, stronger reducing agents being used in substantially light-insensitive thermographic recording materials, the different stabilizers, the different toning agents etc. Furthermore, the thermal development processes themselves are significantly different in that the whole material is heated at temperatures of less than 150° C. for periods of seconds (e.g. 10 s) in the case of photothermographic recording materials, whereas in the case of substantially light-insensitive thermographic recording materials the materials are image-wise heated at much higher temperatures for periods of ms (e.g. 10–20 ms). Realization of a neutral image tone is a major problem in the case of substantially light-insensitive thermographic recording materials due to the very short heating times, whereas it is much less of a problem in photothermographic recording materials due to the much longer heating times.